the true mean is 50 and we reject the hypothesis that μ = 50
The probability of the Type II error will be zero, given that we reject the null hypothesis. This has nothing to do with if it is true or false.
Type II error is occurs when you accept a false null hypothesis. The probability of this error is denoted by beta which relies on sample size and population variance.
The probability of rejecting is equal to one minu beta. i.e it is the researchers goal to reject a false null hypothesis.
